This invention relates to the manufacture of sheet glass by a float process, i.e. by forming molten glass into a continuous ribbon of glass while it is floating on a molten metal, and more particularly to a method of controlling the volumetric rate of flow of molten glass from a glass melting and refining furnace onto a bath of molten metal in order to stabilize the width and thickness of the glass ribbon on the molten metal, the furnace being a regenerative furnace having two sets of oppositely arranged and alternately operatable burner nozzles.
In the manufacture of sheet glass by the float process molten glass is delivered from a glass melting and refining tank furnace onto a bath of molten metal in a forming chamber usually through a canal having a bottom and side walls. The canal is provided with a vertically movable gate called tweel such that an opening through which the molten glass flows is defined between the lower end of the tweel and the bottom of the canal. The volumetric rate of the flow of the molten glass into the forming chamber is primarily governed by the size of this opening, and the opening size depends on the vertical position of the tweel.
In the float process it is favorable to precisely stabilize the width of the glass ribbon formed on the molten metal bath for the enhancement of the yield of sheet glass product. The width of the glass ribbon is affected by the volumetric rate of flow of the molten glass through the aforementioned opening, and some variations occur in the flow rate with variations in the temperature and viscosity of the molten glass in the glass melting and refining furnace. To correct deviations of the glass ribbon width from an aimed value it is customary to control the volumertic rate of flow of the molten glass onto the molten metal bath by minutely controlling the vertical position of the tweel.
For instance, U.S. Pat. No. 4,030,902 shows to find the width of the glass ribbon on the molten metal bath at an appropriate distance from the tweel by detecting the positions of the side edges of the glass ribbon with video-analyzers each of which includes an image pickup tube and, if the found width deviates from an aimed value, regulate the vertical position of the tweel according to the amount of the deviation. JP 59-19887 and JP 59-19888 relate to a tweel position control method of the type shown in U.S. Pat. No. 4,030,902 and propose to regulate the tweel position on the basis of the result of periodical averaging of the detected deviations of the glass ribbon width if the deviations are within a predetermined range but on the basis of the result of periodical sampling and proportional-and-integral treatment of signals representative of the deviations if the deviations are not within that range, and, besides, JP 59-19888 proposes to regulate the tweel position in dependence on the temperature of the molten glass flowing in the canal at a section slightly upstream of the tweel.
Meanwhile, in the industrial practice of the float process usually the glass melting and refining tank furnace is a regenerative furnace having two sets of burner nozzles which are oppositely arranged in the two side walls of the tank furnace, respectively. A regenerator is provided to each set of burners to preheat combustion air with hot exhaust gases, and the two sets of burners are periodically alternately operated with a temporary interruption of firing at each shift of the operation of one set of burners to the operation of the opposite set of the opposite burners.
In our view, when a regenerative furnace of the above described type is used in the manufacture of sheet glass by the float process any of the hitherto proposed tweel position control methods is still insufficient for accurate control the with and thickness of the glass ribbon formed on the molten metal bath.